The prior art is replete with various types of transmissions for off-highway equipment such as tractors and the like. Transmissions for off-highway equipment usually include a casing having a plurality of selectively operable gear meshes that transmit power and torque through various delivery paths between input and output shafts of the transmission. More modem transmissions include two or more constantly intermeshing gears in each gear mesh for developing different speed ratios between the input and output shafts of the transmissions. Some transmissions for off-highway equipment or implements have both a multi-speed transmission section and multi-range transmission section for offering a multitude of various gear ratios capable of developing different ground speeds for the off-highway equipment.
Various gear meshes in the transmission are used to allow the engine to operate so that its output torque follows as closely as possible the maximum power characteristics at the desired ground speed of the implement. In doing so, the transmission varies torque as well as rotational speeds between the input and output shafts. With off-highway equipment, and primarily because of the relatively high draft loads placed on the equipment as they operate in a field, the differences between various gear ratios is relatively small. Thus, the transmission can be conditioned to allow the engine to operate at an optimum speed for different ground speeds of the tractor. Because of the relatively small changes between gear ratios, it is not unusual for an operator to "skip" shifts during the field operation. Under optimum conditions, there is little or no interruption in power during shifting, which contributes greatly to driving comfort, especially when the off-highway equipment is working under a heavy load. To accomplish the goal of little or no interruption in power during shifting, the time required to shift between gear ratios must remain consistent regardless of when the operator decides to shift.
Shifting of a transmission for a tractor and the like is typically carded out by selectively changing which gear meshes in the transmission are coupled to each other as through controlled engagement and disengagement of a series of clutch assemblies. Each clutch assembly in the transmission is operably connected between drive and driven members. Typically, the drive member comprises a gear arranged for rotational movement with a rotatably driven shaft in the transmission and forming part of a first gear mesh. In such an arrangement, the driven member similarly comprises a gear arranged for relative rotation about the shaft on which the drive gear is mounted and forms part of a second gear mesh in the transmission. In one form of the invention, the drive gear may furthermore comprise a clutch housing that rotates with the drive gear.
All transmission clutches further include a common element essential for their functional operation--a piston. The piston is arranged in operable combination with the drive gear and/or the clutch housing to establish an expandable fluid receiving piston cavity or pocket. A typical clutch assembly further includes a clutch pack comprised of a series of interleaved annular friction plates arranged about the rotating shaft and between the drive and driven gears. Some of the annular plates (friction plates) of the clutch pack turn or rotate with the drive member of the clutch assembly while other annular plates (separators) of the clutch pack turn or rotate with the driven member of the clutch assembly. As is conventional, and under the influence of the introduction of fluid into the piston cavity, the clutch piston applies an adequate compressive force against the friction clutch plates such that power and torque are transmitted between the drive and driven members of the clutch assembly. When the clutch assembly is to be released, the flow of pressurized fluid to the piston cavity or pocket is stopped and the clutch piston returns to its released position typically under the influence of a spring.
Pressurized fluid is supplied to the piston cavity through a suitable conduit. The conduit for delivering fluid to the piston cavity or pocket can take many configurations. In many instances, the shaft about which the drive and driven members of the clutch assembly are axially arranged is typically provided with an axially elongated bore that opens to the fluid receiving piston cavity or pocket. A control valve connected to a suitable source of pressurized fluid, such as a pump on the off-highway equipment, controls fluid flow through the conduit and, thus, controls operation of the transmission clutch assembly.
Because of the constant intermeshing relationship between the gears of the gear meshes, or as a result of their positioning within the transmission, clutch assemblies often see "reflected" rotations. That is, although a particular clutch assembly may not be specifically conditioned to establish a drive connection between the drive and driven members, the drive member or gear of the clutch assembly may nevertheless rotate at relatively high speeds. These rotations of the drive gear or drive member, especially at high speeds, can result in inadvertent self-engagement of the clutch assembly as a result of residual centrifugal pressure or centrifugal head force in the piston cavity. As will appreciated by those skilled in the art, inadvertent serf-engagement of the clutch assembly is detrimental to the transmission's logic.
The presence of residual fluid in the piston cavity, the geometrical size of the piston, the speed of rotation of the driven member or gear are all variables which affect the magnitude of the centrifugal head force applied to the piston of the clutch assembly. With increasing speed of rotation of the drive member, the centrifugal head force applied to the clutch piston can be of such magnitude whereby the centrifugal head force overcomes the spring return force and results in inadvertent self-energization of the clutch assembly. As will be appreciated, a supposedly released clutch assembly in the transmission, that becomes serf-engaged will have adverse affects on the logic of the transmission and overall transmission performance. As known, self-energization of the clutch assembly commonly results in increased wear of the clutch plates, significant heat build-up between slipping clutch plates, contamination of the hydraulic fluid, and eventual destruction of the clutch assembly.
Various approaches have been proffered to inhibit the problem associated with centrifugal head force build-up in the piston cavity. One approach to solving serf-energization of the clutch assembly resulting from centrifugal head force involves providing an orifice through a piston wall to allow constant discharge of residual fluid from the piston cavity as long as the piston remains in a released position. The orifice is typically arranged such that a friction plate or reaction member of the clutch pack seals the orifice when the piston of the clutch assembly applies a compressive force to the clutch pack of the clutch assembly. As will be appreciated, and with the piston in a released position, residual fluid flow from the piston cavity through the piston increases proportionately as a function of the rotatable speed of the drive member.
Another approach at reducing the effects of centrifugal head force on the clutch assembly piston that can result in self-energization of the clutch assembly involves installing valve structure in combination with the clutch assembly piston. Like the orifice discussed above, the purpose of the valve structure is to affect the discharge of fluid from the piston cavity as long as the clutch assembly piston is in a released position, thus inhibiting inadvertent application of the clutch assembly.
Various types of valve structures have been proposed for use with the clutch assembly piston. Valves having spring loaded balls are known in the art for controlling the discharge of hydraulic fluid from the piston cavity as long as the clutch piston remains in a released position. Also, valve structures with housings having conically angled ramps on which a variably weighted ball rides are known in the art. In the later version, the size of the ball and the angle Of the ramp are calculated and selected according to the specific needs of a given clutch assembly application. When the piston cavity is pressurized, the ball of the valve structure seals the piston cavity thus making clutch energization short and efficient.
Heretofore known devices for dissipating centrifugal head force in a piston cavity of a clutch assembly also have certain drawbacks. The size of the opening in the piston, and/or the size of the detent or ball comprising part of such heretofore known valve structures, are relatively small. Accordingly, contaminants commonly found in the transmission fluid often block the orifice or interfere with proper operation of the valve structure and, therefore, the problems of self-energization of the clutch assembly remains a serious concern.
As an operator drives the equipment across a field, and because the change effected between consecutive gear ratios of off-highway equipment is relatively small, the transmission typically remains in a constant condition. As such, the drive members operably associated with various non-engaged clutch assemblies continue to rotate at relatively high rotational speeds. As mentioned, the heretofore known devices for dissipating centrifugal head force in the various piston chambers of the non-engaged clutches allow residual fluid to be discharged from the piston chamber to inhibit self-energization of the clutch assembly.
Although no fluid flow is specifically directed to the non-engaged clutch assemblies, upon disengagement of the clutch assembly residual fluid normally remains in the conduit or passage leading to the piston cavity. The heretofore known devices for dissipating centrifugal head force in the piston cavity are known to drain not only the piston cavity of the non-engaged clutch assembly, they furthermore drain residual fluid from the supply conduit leading to the piston cavity thus leaving an "air space" in the fluid supply conduit or line. Thus, when a non-engaged clutch assembly is eventually engaged, pressurized fluid must travel through and initially fill a drained supply line or conduit before the pressurized fluid is presented to and fills the piston cavity of the to-be-engaged clutch assembly. The need to fill both the supply line and the piston cavity with pressurized fluid leads to unpredictability of clutch performance.
Because of the variables involved, for example, the speed of rotation of the driven member or gear operably coupled to the clutch assembly, the time when the operator decides to shift or change conditions of the transmission, and the temperature condition of the hydraulic transmission fluid, it is nearly impossible to predict the "fill" status of various supply conduits and piston cavities prior to and during a transmission shift. Consequently, each shift or change in transmission condition will be different thus degrading shift quality while adversely effecting power transfer, especially under heavy draft load conditions, for the implement thereby affecting driving comfort for the operator.
Thus, there is a need and a desire for a transmission clutch assembly including structure for eliminating centrifugal head force in a non-engaged clutch while affording substantially constant shift times regardless of when the operator decides to shift or change the condition of the transmission.